This section provides background information related to the present disclosure which is not necessarily prior art.
A model of a typical prior art ultrasonic metal welding apparatus 100 is shown in FIG. 1. Typical components of ultrasonic metal welding apparatus 100 include an ultrasonic transducer 102, a booster 104, and an ultrasonic horn 106. The ultrasonic transducer 102, booster 104 and ultrasonic horn 106 comprise weld stack 118. It should be understood that in some cases, weld stack 118 does not have booster 104. Electrical energy from a power supply 101 at a frequency of 20-60 kHz is converted to mechanical energy by the ultrasonic transducer 102. The mechanical energy converted in the ultrasonic transducer 102 is transmitted to a weld load 108 (such as two pieces of metal 112, 114) through the booster 104 and the horn 106. The booster 104 and the horn 106 perform the functions of transmitting the mechanical energy as well as transforming mechanical vibrations from the ultrasonic transducer 102 by a gain factor.
The mechanical vibration that results on a horn tip 110 is the motion that performs the task of welding metal together. Horn tip 110 may be made of tungsten carbide or other high strength, hard material. The metal pieces 112, 114 to be welded together are placed adjacent to the horn tip 110. The horn tip 110 is brought into contact with top metal piece 112 to be welded. The axial vibrations of the ultrasonic horn 106 now become shear vibrations to the top metal piece 112. The shear vibrations are transmitted to the top metal piece 112, causing it to move back and forth with respect to bottom metal piece 114 causing surfaces of the two metal pieces abutting each other at a weld interface to be heated, eventually melting together. A weld anvil 120 grounds the bottom metal piece 114. It should be understood that such an ultrasonic welder can be used to weld multiple metal foil layers together, such as several layers of aluminum or copper foil.
A similar apparatus is used in ultrasonically welding plastic pieces together. The principal difference is that the ultrasonic horn oscillates in a manner to impart vertical oscillations in the plastic pieces. That is, the ultrasonic horn causes oscillatory compression/decompression of the plastic pieces with respect to each other causing surfaces of the plastic pieces abutting each other at a weld interface to be heated, eventually melting together.
Ultrasonic welders are for example disclosed in U.S. Pat. No. 5,658,408 for Method for Processing Workpieces by Ultrasonic Energy;“ U.S. Pat. No. 6,863,205 for Anti-Splice Welder,” and US Pat. Pub. No. 2008/0054051 for “Ultrasonic Welding Using Amplitude Profiling.” The entire disclosures of the foregoing are incorporated herein by reference.
One type of ultrasonic horn is sometimes referred to as a slotted ultrasonic block horn. This type of ultrasonic horn is made of a block of metal, such as steel, aluminum or titanium, with slots machined in it between the top and bottom surfaces. Typical slotted ultrasonic block horns resonant at ultrasonic frequencies typically used in ultrasonic welders (e.g., 15 kHz-60 kHz) have higher than desired quality (“Q”) factors. Q factor is the resonant frequency of the ultrasonic horn divided by the bandwidth of the ultrasonic horn. The higher the Q factor, the narrower the bandwidth and vice-versa. Using ultrasonic horns with these high quality factors lead to ultrasonic stacks that have very low bandwidth. Low bandwidth ultrasonic stacks are problematic for ultrasonic power supplies to track and maintain axial resonance during typical weld cycles. This can result in overloads of the ultrasonic power supplies due to the difficulties in tuning and tracking the resonant frequency.